Without limiting the scope of the invention, its background is described in connection with evaporative coolers are a popular choice for HVAC (heating/venting/air conditioning) service, especially in dry climates, as they can simultaneously cool and humidify the air, and do so with considerably less electrical power consumption than conventional refrigerant systems using fluorocarbon refrigerants. However, evaporative coolers have several problems not present with refrigerant systems, including scale build-up and the growth of mold, algae and other microbes. These problems require regular maintenance, adding to the cost of operation. The added cost of maintenance in some cases can outweigh the cost benefit of lower electrical consumption.
Water used in evaporative coolers ordinarily contains dissolved minerals such as carbonates, sulfates, and nitrates of calcium, magnesium, potassium and sodium, which deposit on the contact media as scale. As the water evaporates, the concentration of dissolved minerals increases, causing more rapid scale build-up on the contact media and the formation of particulates in the water. Scale tends to reduce the evaporative efficiency of the contact media, and will eventually clog the passages through which the water and air pass, further reducing evaporator efficiency. Moreover, the added weight from the scale deposits can cause deformation or collapse of insufficiently supported media. Depending on the makeup of ionic material dissolved in the water, the water may become acidic or alkaline, which can to also promote deterioration of the contact media. Mold, algae and mildew can also develop that attack the contact media, create objectionable odors and present a potential health hazard.
Several methods have been used to address the problems of scale build-up on the contact media: 1) use of once-through water or use of recirculating water with a high bleed-off water rate to reduce the concentration of dissolved salts; 2) addition of scale inhibiting chemicals to the recirculated water; and 3) use of untreated recirculated water without bleed-off, along with periodic replacement of the contact media. All of these methods add to the operating and maintenance costs. If a replaceable contact media can be made that is long lasting and inexpensive however, the third method becomes attractive.
Replaceable contact media has been made with cellulose, asbestos, or fiberglass sheets. These materials are preferred for their large effective surface area and good wetting properties, which promotes greater evaporation rates for a given amount of material. However, materials having these desired properties often also lack the needed rigidity and water resistance to hold up under typical service for extended periods.
To improve the longevity of the contact media, it is common to impregnate the bulk material with a polymer material. Impregnation can increase the overall structure's strength, especially when wet, and thereby increase its durability and resistance to deformation caused by scale build-up. Different organic and inorganic materials have been used, with organic polymers being a popular choice.
U.S. Pat. No. 3,262,682, issued to Bredberg and U.S. Pat. No. 3,792,841, issued to Munters, teach impregnating cellulose or asbestos sheets with either a phenolic aldehyde resin or a phenolic resin to increase wet strength. Other polymers commonly used in the industry are urea formaldehyde, melamine, and melamine formaldehyde, all of which are thermosetting plastics that are cured on the bulk material. Unfortunately, these polymers tend to break down under contact with acidic or alkaline recirculated water, hydrolyzing back into the original reactants and other smaller compounds that dissolve and are washed away, leaving the bulk material unprotected and unsupported. Some of the hydrolysis products are volatile and will vaporize and be blown into the ventilation ducting along with the cooled air, polluting the air in the living space. The remaining, environmentally harmful hydrolysis products remain dissolved in the water, and are usually dumped into the local water table when the cooler is flushed out, because the environmental hazard created by this type of contact media is not generally recognized.
U.S. Pat. No. 3,798,057 and U.S. Pat. No. 3,862,280, both issued to Polvina, disclose the use of a special bulk material that is acid, alkali, and water resistant, impregnated with a combination of a chlorinated (3 or (5 hydrocarbon, a chlorinated terphenyl or chlorinated paraffin (as a plasticizer), and a polyglycidyl ether polyhydric phenol such as bisphenol A or bisphenol F. This impregnating material is claimed to increase durability under pH and temperature extremes that normally cause rapid disintegration of conventional contact materials.
While all the foregoing impregnating methods offer certain advantages, they also have significant drawbacks. The polymers in the prior art are anionic, meaning that they attract positively charged particles or ions, which include the dissolved metals previously discussed. Thus, these polymers aggravate scale build-up which shortens the media's useful life span. In addition, most of these polymers have values of interfacial tension that are only a fraction of the value for water, resulting in a large interfacial tension between the surface of the polymer and the water. This means that the water will not be able to wet the polymer as well as will more compatible polymers, which in turn means these materials will evaporate water at a lower rate than untreated material, given the same operating conditions and media size.
International Patent Application W091 03778 (hereafter IPA '778), filed by Myers et al., and U.S. Pat. No. 5,260,117, issued to Meyers et al., teaches impregnating a honeycomb structure with various thermosetting polymers to improve the structure's mechanical properties. A critical feature of the teachings is that the polymer precursors are dissolved in a solvent that does not dissolve the resulting polymer. The honeycomb is dipped in the solution, then heated in an oven which evaporates the solvent and is claimed to cause the precursors to react and form the final thermosetting polymer by homolineation, a reaction that results in long unbranched polymer chains without crosslinking. (The IPA '778 mistakenly describes the polymers as thermoplastic in nature, and makes numerous other errors, ascribing several characteristics to the polymer that are present in thermoplastic polymers, but not in thermosetting polymers, such as absence of crosslinking. These mistakes are substantially but not completely corrected in U.S. Pat. No. 5,260,117 [e.g. continued requirement for the absence of crosslinking in thermosetting polymers; separately applied thermosetting polymer layers are said to “fuse” together when the layers actually adhere to each other].) The polymer homologs taught in the Meyers et al. references are selected on the basis of mechanical properties such as strength, impact resistance and surface finish and appearance; there is no discussion of chemical properties such as wettability, ionic behavior, and solubility in water. Both Meyers et al. references only discuss solubility with respect to the solvent, and only polar organic solvents are specifically listed. Analysis of Meyers reveals that many of the polymer homologs will exhibit the undesirable interfacial tension and anionic behavior of the previously discussed prior art polymers; some of the listed polymers will also have hydrolysis decomposition problems.
A desirable replaceable contact media will have relatively high water resistance (i.e., low solubility in water) and retain its strength when wet. The contact media should also resist scale build-up and have improved wetting properties relative to conventional polymers for greater evaporative rates. The contact media preferably will also resist growth of mold, algae, mildew and other microbes. The media should retain these properties and resist chemical breakdown in the presence of acidic or alkaline conditions. As always, a contact media that is less expensive to manufacture is also desired.